Lewis Hamilton's Mercedes team were dealt a blow on the opening morning of Formula One testing after they were sidelined with a mechanical failure.

The world champions were absent for the majority of the four-hour running at the Bahrain International Circuit following a gearbox failure.

Hamilton's team-mate Valtteri Bottas eventually emerged from the Mercedes garage with 35 minutes remaining and completed only six laps, finishing last of the 10 runners.

Hamilton, who is this season bidding to win an unprecedented eighth world title, will take over the driving duties from Bottas following the one-hour lunch break.

The three-day test in Bahrain, which concludes on Sunday, resembles the only running before the season gets under way at the Gulf Kingdom on March 28.

Mileage is king in testing and Mercedes are now on the backfoot with Red Bull's Max Verstappen, who is expected to be the closest challenger to the Silver Arrows, completing 60 laps.

Daniel Ricciardo topped the time charts in his McLaren, finishing 0.028 seconds clear of AlphaTauri's Pierre Gasly – the Frenchman racking up 72 laps – with Verstappen in third, 0.042 sec off the pace.

"It was really good and a nice first half of the day," said Ricciardo, who has joined McLaren from Renault. "Everything went smoothly and it is nice to see your name up the top.

"But testing, especially day one, is about reliability and making sure everything runs. The team is happy and it is nice to get back into it."

Sebastian Vettel broke down at the end of the pit lane in his Aston Martin before finishing seventh, while Mick Schumacher also ran into trouble.

Schumacher, the son of seven-time world champion Michael Schumacher, managed only 15 laps following a gearbox change.

Ferrari's Charles Leclerc spluttered to a stop with 10 minutes remaining bringing out the first red flag of the day.

The Italian team are bidding to bounce back from their worst season in four decades after they finished a lowly sixth in last season's constructors' championship.